LNG vaporization is a relatively energy intensive process, which typically requires a heat duty equivalent to about 1.5% of the energy content in the LNG. This significant energy demand is often met by use of seawater as an external heat source. However, regasification of LNG using seawater is environmentally problematic due to the local reduction of water that adversely affects sea life cycles and habitats. Alternatively, LNG can be vaporized using fuel gas combustion in submerged combustion vaporizers. However, such system requires fuel gas and tends to create undesirable emissions.
In still further known methods of regasifying liquefied gases, ambient air is used as a heat source in an ambient air vaporizer. While this type of vaporizer is fairly common in cryogenic plants (e.g., nitrogen vaporizers), ambient air vaporizers are generally limited to relatively small capacity and will in most cases fail to produce an ambient temperature gas product from LNG. When applied to LNG vaporization, ambient air vaporizers can typically produce a vaporized product at about −40° F., which is not acceptable to most pipeline specifications. To increase the temperature of vaporized LNG, heat is supplemented to an ambient air vaporizer as taught in U.S. Pat. No. 7,392,767. Here, a quench column is employed as a heat source for heating of a circulating liquid, and an additional indirect heat exchanger is employed to produce a hot liquid stream that supplies heat to vaporize the LNG. While such quench column configuration can be effective in some waste heat recovery configurations, particularly from waste heat from the gas turbine exhaust, over-chilling of the exhaust gas to a temperature below the dew point of the exhaust gas is unavoidable, resulting in undesirable waste by-products that must be neutralized or otherwise disposed of.
Alternatively, as described in U.S. Pat. No. 5,251,452, LNG ambient air vaporizers can be operated using cyclic heating and defrosting. While such vaporization is conceptually simple and does not require LNG as a fuel source, various disadvantages nevertheless remain. Among other things, such ambient air vaporization schemes generally require a relatively large number of air vaporizers, plot space, and consequently high operating and capital cost. To circumvent at least some of the above drawbacks, various new configurations have been described that help recover power at the LNG receiving terminal. In such configurations, LNG is used as a heat sink for power generation, and/or as fuel to a power plant as described in our copending International patent applications with the serial numbers PCT/US03/25372 (published as WO 2004/109206 A1), PCT/US03/26805 (published as WO 2004/109180 A1), and PCT/US05/24973 (published as WO 2006/019900 A1), all of which are incorporated by reference herein.
While most of these configurations tend to reduce energy consumption to at least some extent (e.g., via ambient air vaporizers and/or waste heat recovery from gas turbine exhaust), such configuration are still relatively inefficient often generate liquid effluent without significant improvement in power generation efficiencies. In further known configurations, as described in U.S. Pat. No. 5,457,951, LNG is regasified in an integrated combined cycle power plant using a heat transfer medium where the working fluid of the steam cycle is in thermal exchange with a heat exchange fluid of the heat exchanger in which the LNG is vaporized and wherein the heat exchange fluid further chills the intake air of the compressor. However, such configurations are typically limited to conventional heat exchangers and therefore fail to take advantage of the benefits of ambient air exchangers.
Therefore, while numerous processes and configurations for power plants with LNG utilization and/or regasification are known in the art, all of almost all of them suffer from one or more disadvantages. Thus, there is still a need to provide improved configurations and methods for waste heat recovery in the area of ambient air vaporizers in LNG regasification.